Process for the mixed production of high and low purity oxygen

ABSTRACT

A process for simultaneously providing both high and low purity oxygen from a double column, double reboiler apparatus, the double columns being high and low pressure columns, the low pressure column having a lower and upper reboiler with an intermediate outlet in between, which process entails extracting a low purity oxygen stream at the intermediate outlet and a high purity liquid oxygen stream from a bottom portion of the low pressure column.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the mixed production ofhigh or low purity oxygen using a double reboiler and double columnarrangement.

2. Description of the Background

The double reboiler and double column process was developed to producerelatively low purity oxygen at purities below 97% oxygen content. Thisprocess is described in U.S. Pat. No. 3,113,854 and in the ElectricPower Research Institute (ERPI) Study 3499. Additionally, U.S. Pat. No.4,895,583 discloses a modification of an earlier process in order toimprove the overall recovery of oxygen.

Another example of a conventional double column and double reboilerprocess for producing relatively low purity oxygen is that disclosed inU.S. Pat. No. 4,705,548. However, this process is disadvantageous as itrequires the use of a compressor for nitrogen recycling.

Attempts to produce high oxygen purities, such as above 97% oxygencontent, using the double reboiler and double column methodology havenot been considered interesting due to the sharp reduction in processefficiency and loss of recovery which is entailed. These drawbacks occurbecause it is necessary to perform the very difficult argon/oxygenseparation by cryogenic distillation for oxygen contents of higher than97%.

At present, in order to obtain an oxygen content of greater than 97%,the single reboiler, double column process is usually used.Unfortunately, this process has a much higher specific power consumptionthan the double reboiler and double column process.

It would be extremely advantageous if a process were known for producingoxygen at both low and high purity. Frequently, an on-site oxygenfacility may produce a gaseous oxygen product at 95% purity for apipeline customer, yet local demand may require a liquid oxygen producthaving 99.6% oxygen content which is a normal standard grade for liquidoxygen.

Furthermore, in situations where significant liquid production isrequired a traditional single reboiler process is used whereby air,expanded through an expander, is injected into the high pressure column.This affords a good recovery of oxygen since the majority of the air isstill being treated in the high pressure column and does not by-pass thefirst step of distillation.

However, this feature has a major drawback because of the relativelyhigh discharge pressure of the expander which is the high pressurecolumn pressure. The feed air pressure must, therefore, be maintained ata high pressure to achieve the required refrigeration. In suchsituations, high power consumption would be needed to produce therequired liquid.

Thus, a need exists for a process for producing oxygen at both low andhigh purity, more precisely, low purity gaseous oxygen and high purityliquid oxygen, and which is not characterized by a prohibitively highspecific power consumption.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aprocess for producing oxygen at both low and high purity.

It is also an object of the present invention to provide a process forproducing oxygen at both low and high purity, which is not characterizedby having a prohibitively high specific power consumption.

Moreover, it is a specific object of the present invention to producesimultaneously both high and low purity oxygen using a double reboiler,double column process and optionally with feed stream expansion into thehigh pressure fractionating means.

The above objects and others which will become apparent in view of thefollowing disclosure, are provided by a process of simultaneouslyproducing both high and low purity oxygen, which entails:

a) passing a compressed and cooled feed stream containing at leastoxygen and nitrogen to a main exchanger, wherein the compressed andcooled feed stream is divided into two fractions (A) and (B),

b) expanding the first fraction (A) in an expander and dividing the sameinto two further fractions (i) and (ii),

c) further cooling the second fraction (B) in the main exchanger andexpanding at least a fraction of this stream by an expansion valve intoa high pressure column as liquid feed stream,

d) introducing the first fraction (i) into a reboiler located at thebottom of the low pressure distillation column, whereby the firstfraction of the feed stream condenses in the reboiler providing a firstreboil for distillation,

e) feeding at least a fraction of the resulting liquefied feed stream toboth high and low pressure columns as feed,

f) introducing the second fraction (ii) to the bottom of the highpressure column as gaseous feed, whereby a bottom oxygen-rich liquidstream and an overhead nitrogen-rich gaseous stream are formed,

g) passing the oxygen-rich liquid stream to the low pressure column asfeed,

h) condensing the nitrogen-rich gaseous stream against a boiling liquidin the upper reboiler of the low pressure column, located at anintermediate stage, thereby serving as a second reboil for distillation,

i) returning a portion of the condensed nitrogen-rich stream to the highpressure column as reflux, and passing at least a fraction of theremaining portion of the nitrogen-rich stream to the top of the lowpressure column as reflux, whereby the low pressure column separates itsfeed into bottom stream oxygen products and top stream rich in nitrogen,

j) recovering the bottom stream oxygen product as high purity liquidoxygen,

k) drawing from a tray or a stage between the two reboilers of the lowpressure column, the stream oxygen product, as low purity oxygen, and

l) warming in the main exchange the low purity oxygen stream, and thenitrogen-rich stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relationship between overall oxygen recovery andpercentage of high purity liquid produced for 95% low purity gaseousoxygen and 99.9% high purity liquid oxygen.

FIG. 2 illustrates an apparatus for practicing the present invention.

FIG. 3 illustrates an alternative apparatus for practicing the presentinvention using a pumping process where pumped liquid oxygen isvaporized directly in the main exchanger.

FIG. 4 illustrates an apparatus for practicing the present inventionusing a pumping process where pumped liquid oxygen is vaporized in aseparate exchanger against a condensing feed stream.

FIG. 5 illustrates an apparatus for practicing the present invention andrecovering argon using an argon side arm column at a tray or stagebetween two oxygen product outlets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, both high and low purityoxygen are simultaneously produced using a double reboiler, doublecolumn process with feed stream expansion having reduced powerconsumption.

In more detail, it has been discovered that by adding an intermediateoutlet between the two reboilers of the upper distillation column, a lowpurity oxygen stream can be extracted at this outlet simultaneously withanother high purity liquid oxygen stream produced at the bottom of thedistillation column. By using the present invention, significant amountsof high purity liquid oxygen can be produced simultaneously with the lowpurity oxygen product with excellent recovery/efficiency.

The double reboiler, double column process is characterized bycondensing a feed stream against oxygen product. This results in arelatively low operating pressure of about 4 bar for the high pressurecolumn. A conventional single reboiler process would require a pressureof about 6 bar. Therefore, significant power savings are achievable withthe double reboiler, double column process if an oxygen purity of lessthan 97% is acceptable.

However, poor oxygen recovery would result if higher purity oxygen isrequired using the double reboiler, double column process.

That is, the double reboiler, double column process is characterized bya low operating pressure, low purity oxygen and low power consumption.In contrast, the traditional simple reboiler process is characterized bya high operating pressure, high purity oxygen and high powerconsumption.

In order to produce a significant amount of liquid oxygen, the feedstream expansion process is frequently used where the feed stream isexpanded through an expander into the high pressure column. For theproduction of a given amount of liquid, feed stream pressure upstream ofthe expander would be much lower for the double reboiler process sinceits discharge pressure is the lower of the two processes. Therefore,much lower power consumption can be achieved for the liquid production.

In accordance with the present invention, it has been surprisinglydiscovered that the double reboiler process can produce a highproportion of high purity oxygen product while maintaining high overalloxygen recovery. For example, up to 30% of 99.9% purity liquid oxygencan be produced in conjunction with 95% purity gaseous oxygen and with arecovery of between 99.5% and 97.5%, which is, indeed, unexpected. Thisresult is unexpected because 30% is a high proportion for a cryogenicair separation process.

Of course, the process of feed stream expansion into a high pressurefractionating means is suitable for relatively significant liquidproduction at 40% or below of the total production. For higherpercentages, one must rely upon an independent refrigeration cycle. Forsmall quantities of liquid production, any expansion means is suitable,such as nitrogen-rich gas expansion or air expansion into a low pressurefractionating means.

As used herein, the term "feed stream" refers to any gaseous mixturecontaining at least nitrogen and oxygen. For example, the feed streammay be atmospheric air or may be an off-gas mixture.

As used herein, the term "recovery" is defined as total contained oxygenin products divided by total oxygen contained in the feed stream. Theterm "percent high purity oxygen" is defined as production of highpurity oxygen divided by total oxygen production.

Furthermore, as used herein, the term "high purity oxygen" means any gascontaining at least 97 mol% of oxygen, whereas the term "low purityoxygen" means any gas containing less than 97 mol% of oxygen.

The present invention provides an intermediate outlet between the tworeboilers of the upper distillation column to extract a low purityoxygen stream. Additionally, a high purity liquid oxygen stream isrecovered at the bottom.

In particular, the present method involves various steps which will nowbe described in general by reference to FIG. 2.

First, a feed stream, such as purified atmospheric air, is compressedand cooled. Thereafter, the feed stream is fed through conduit (101) toa main exchanger (11), where the feed stream is further cooled and thendivided into two fractions (A) and (B). First fraction (A) is thenexpanded in expander (12), and thereafter further divided into twofractions (i) and (ii) at the junction of conduits (106) and (107).

Feed stream (B) is further cooled in the main exchanger and at least afraction of this stream is expanded by expansion valve (130a) into ahigh pressure column (20) as liquid feed stream.

First fraction (i) is introduced into a reboiler (24) at the bottom ofthe low pressure distillation column (21) to provide a first reboil fordistillation. Thereafter, at least a fraction of the resulting liquefiedfeed stream is fed to either or both high and low pressure columns asfeeding conduits (111) and/or (112).

Then, the second fraction (ii) is fed to the bottom of the high pressurecolumn (20) as gaseous feed, to form a bottom oxygen-rich liquid stream(108) and an overhead nitrogen rich gaseous stream (114). Thereafter,the oxygen-rich liquid stream is passed to the low pressure column (21)as feed.

The nitrogen-rich gaseous stream is condensed against a boiling liquidin the upper reboiler (23) of the low pressure column (21), therebyserving as a second reboil for distillation. Then, a portion of thecondensed nitrogen-rich stream is returned in conduit (131) to the highpressure column (20) as reflux via conduit (115) and the remainingportion of the nitrogen-rich stream to the top of the low pressurecolumn as reflux via conduit (116), whereby the low pressure column (21)separates its feeds into bottom stream oxygen products and top stream(120) rich in nitrogen.

Thereafter, bottom stream oxygen product (118) is recovered ashigh-purity oxygen. A stream (111) is then drawn at a stage between thetwo reboilers (23) and (24) of the low pressure column (21) as lowpurity gaseous oxygen product. Finally, the nitrogen-rich stream iswarmed in at least exchanger (11), and optionally exchanger (22), viaconduits (120) and (121), with the low purity oxygen stream (119) fromthe low pressure column.

Additionally, in FIG. 2, a booster compressor (10), driven by theexpander (12), may be optionally used to further compress the feedstream entering via conduit (100). The feed stream then is fed viaconduit (101) to the main exchanger (11) and therethrough via conduit(102), whereinafter a portion of the feed stream is passed via conduit(103) to expander (12) and then to conduit (105), and thereinafterfurther divided into two fractions via conduits (106) and (107) asdescribed above.

Further, in FIG. 2, oxygen-rich liquid stream (108) is optionallysubcooled in exchanger (22 and then transferred to the low pressurecolumn (21) via conduit (109). Moreover, the liquefied feed stream (112)ma be subcooled in exchanger (22), and then transferred to the lowpressure column via conduit (113). However, the liquefied feed streammay be fed directly to the low pressure column.

In more detail, a feed stream, containing at least nitrogen and oxygen,which has been compressed and cooled is passed to a main exchanger. Thefeed stream is compressed in compressing means and cooled. For example,the compressing means may comprise a booster (10) driven by a turbine(12). Generally, the feed stream after compression is at a pressure inthe range of about 6 to 50 bar. The preferred pressure depends upon thepercentage of liquid produced. If a higher percentage of liquid isneeded then higher pressure is required. However, in general, thepreferred pressure is in the range of about 10 to 35 bar. Then, thecompressed, cooled and purified feed stream is further cooled in a mainexchanger, where it is divided into two fractions (A) and (B).Generally, the feed stream is at a temperature in the range of about-120° C. to -150° C. It is preferred, however, that the feed stream beat a temperature of about -125° C. to -145° C. Thereafter, the firstfraction (A) is expanded in step b) in an expander and subsequentlydivided into two further fractions (i) and (ii). Generally, thepreferred discharge pressure range of the expander is 3 to 5 bar. Then,the second fraction (B) is further cooled in step c) in the mainexchanger and at least a fraction of this stream is expanded by anexpansion valve into the high pressure column as liquid feed stream.

In step c), fraction (B) is generally subcooled beyond the bubble point.It is preferred, however, that it be subcooled to a temperature in therange of about -150° C. to -180° C.

Then, in step d), the first fraction (i) is introduced into a reboilerlocated at the bottom of the low pressure distillation column, wherebythe feed stream condenses in the reboiler providing a first reboil fordistillation. At least a fraction of the resulting liquefied feed streamis then fed to either or both high and low pressure columns as feed. Thetemperature of the resulting liquefied feed stream to the high pressurecolumn is generally near the bubble point, preferably at the bubblepoint. The temperature of the liquefied feed stream to the low pressurecolumn is generally subcooled below the bubble point. The secondfraction (ii) is introduced to the bottom of the high pressure column asgaseous feed, whereby a bottom oxygen-rich liquid stream and an overheadnitrogen-rich gaseous stream are formed. The temperature is near the dewpoint. The oxygen-rich liquid stream is then passed to the low pressurecolumn as feed.

Generally, in passing the oxygen-rich liquid stream to the low pressurecolumn as feed, the stream is preferably subcooled before expanding intothe low pressure column.

The nitrogen-rich gaseous stream is then condensed against a boilingliquid in the upper reboiler of the low pressure column, thereby servingas a second reboil for distillation. A portion of the condensednitrogen-rich stream is returned to the high pressure column as reflux,and the remaining portion of the nitrogen-rich stream is passed to thetop of the low pressure column as reflux, whereby the low pressurecolumn separates its feeds into bottom oxygen products and top streamrich in nitrogen.

Then, the bottom oxygen product (118) is recovered as high purity liquidoxygen. Generally, for the bottom stream oxygen products purities in therange of 97% to 99.99% are obtained. Thereafter, a stream (119) is drawnat a tray located between the two reboilers of the low pressure columnas low purity oxygen. In this step, the vapor product can be extractedas part of the vapor leaving the tray or stage in the case thatstructured packings are used or, an equivalent means which isconventionally so used. If the product is liquid, the liquid product isextracted as part of the liquid leaving the tray or stage. Finally, inthe main exchanger, the low purity oxygen stream and the nitrogen-richstream are warmed to ambient conditions.

However, in addition to the exemplary process scheme described above,various modifications may also be made. For example, argon may berecovered by means of an argon sidearm column at a tray between the twooxygen product outlets or a stage in the case of structured packings.

As used herein, the term "tray" refers to any means or device foreffecting intimate contact and mass transfer between a descending liquidphase and an ascending vapor phase. These various trays are well knownto those skilled in the art.

The term "tray" as used herein also includes means known as structuredpackings or equivalent means which are devices equivalent to trays toeffect such intimate contact for oryogenic air separation. Examples ofstructured packings are disclosed, e.g. in U.S. Pat. Nos. 2,047,444;4,186,156 and 4,296,050, each patent being incorporated herein byreference in the entirety, and Ellis et al, Trans. Instn. Chem. Engrs.,41, 1963, known as Goodloe packings. These structured packings are knownas means to promote liquid and/or vapor mixing in a directionperpendicular to the primary flow direction, i.e., the verticaldirection. Furthermore, a combination of trays and packings can be usedas mass transfer means.

Reference will now be made to each of FIGS. 1-4 in detail.

FIG. 1 illustrates the relationship between overall oxygen recovery andpercentage of high purity liquid produced for 95% low purity gaseousoxygen and 99.9% high purity liquid oxygen.

As may be seen from FIG. 1, recovery remains rather constant up to about30% high-purity liquid oxygen production, and drops sharply beyond thispoint. Generally, in accordance with the present invention, recoveriesof high purity liquid oxygen of equal to or less than 35% are obtained.

FIG. 2 illustrates a embodiment of the present invention as describedabove.

FIG. 3 illustrates another embodiment of the present invention using apumping process where pumped liquid oxygen is vaporized directly in themain exchanger.

In FIG. 3, the low purity oxygen stream (119) is produced in the liquidform directly off the low pressure column (21). This liquid is thenpumped to a higher pressure using a pumping means (140) and thenvaporized and warmed in the main exchanger (11). At the same time, ahigh pressure feed stream (104) is condensed in the main exchanger (11)to provide necessary heat for the vaporization of the oxygen stream.

The remaining elements depicted in FIG. 3 correspond to those alreadydepicted and described for FIG. 2.

In FIG. 4, the low purity oxygen stream (119) is produced in the liquidform directly off the low pressure column (21). This liquid is thenpumped to a higher pressure using a pumping means (140) and thenvaporized in separate exchanger (150) against a condensing feed stream.

The remaining elements depicted in FIG. 4 correspond to those alreadydepicted and described for FIG. 2.

In FIG. 5, an apparatus is shown for practicing the present inventionand recovering argon using an argon side arm column at a tray or stagebetween two oxygen product outlets. Notably, the argon side arm columnis shown as element (141).

In accordance with another aspect of the present invention, a process isprovided for simultaneously producing both high and low purity oxygen,which entails:

a) condensing a first compressed, cooled and purified feed streamfraction in a reboiler located at the bottom of a low pressure column;

b) feeding at least a fraction of the resulting liquefied feed stream toeither or both high and low pressure columns as feed;

c) introducing a second fraction of feed stream to the bottom of thehigh pressure column as gaseous feed, whereby a bottom oxygen-richliquid stream and an overhead nitrogen-rich gaseous stream are formed;

d) passing the oxygen-rich liquid stream to the low pressure column asfeed;

e) condensing the nitrogen-rich gaseous stream against a boiling liquidin the upper reboiler of the low pressure column, thereby serving as asecond reboil for distillation;

f) returning a portion of the condensed nitrogen-rich stream to the highpressure column as reflux, and passing at least a fraction of theremaining portion of the nitrogen-rich stream to the top of the lowpressure column as reflux, whereby the low pressure column separates itsfeeds into bottom stream oxygen products and top stream rich innitrogen;

g) recovering the bottom stream oxygen product as high purity liquidoxygen; and

h) drawing from a tray or a stage between the two reboilers of the lowpressure column, the stream oxygen product as low purity oxygen.

In the above process, the temperatures and pressures recited for theprevious process are also applicable therein.

As used herein, the term "feed stream" refers to any mixture of gascontaining nitrogen and oxygen, such as air or any off-stream gasmixture containing oxygen, nitrogen and other gases. In the presentinvention, however, it has been found advantageous to use atmosphericair, which has been cleaned and dried, as a feed stream. Further, thepresent process is of particular advantage when used with any feedstream mixture containing nitrogen, oxygen and argon.

The present invention also provides various apparati for practicing theprocesses of the present invention.

Generally, the present invention provides an apparatus forsimultaneously producing both high and low purity oxygen, which entailsa double rectification column having a lower pressure fractionatingmeans with two reboilers therein, and a higher pressure fractionatingmeans, wherein an intermediate outlet is provided between the tworeboilers of the lower pressure fractionating means for simultaneouslyextracting a low-purity oxygen stream and a high-purity liquid oxygenstream.

In more detail, the apparatus entails a double rectification columnhaving high and low pressure fractionating means with two reboilers inthe low pressure fractionating means, and having a first feed streaminput means in fluid connection with a first reboiler located at thebottom of the low pressure fractionating means, a second feed streaminput means in fluid connection with the bottom of the high pressurefractionating means, and a third feed stream input means in fluidconnection with the high pressure column; expanding means in contactwith the second and third feed stream input means and being upstream ofthe double rectification column; conduit means for feeding liquefiedfeed stream to both high and low pressure fractionating means from thelower reboiler of the low pressure fractionating means; conduit meansfor feeding part of a nitrogen-rich stream from the upper reboiler ofthe low pressure fractionating means to the high pressure fractionatingmeans, and conduit means for feeding a remaining part of thenitrogen-rich stream to the top of the low pressure fractionating means;conduit means for feeding an oxygen-rich liquid stream to the lowpressure fractionating means; and output means for recoveringhigh-purity oxygen. Such an apparatus is depicted in FIG. 2, and hasalready been described in detail above.

Alternatively, an apparatus is provided wherein the low purity oxygenstream is produced in liquid form directly off the low pressure column.This liquid is then pumped to a higher pressure using pumping means andthen vaporized and warmed in the main exchanger. At the same time, ahigh pressure feed stream is condensed in the main exchanger to providenecessary heat for the vaporization of the oxygen stream. Such anapparatus is depicted in FIG. 3.

Additionally, an apparatus is provided wherein the low purity oxygenstream is produced in the liquid form directly off the low pressurecolumn. This liquid is then pumped to a higher pressure using pumpingmeans and then vaporized in a separate exchanger against a condensingfeed stream. Such as apparatus is depicted in FIG. 4.

Having described the present invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade to the above-described embodiments without departing from thespirit and the scope of the present invention.

What is claimed as new and desired to be secured by letters patent ofthe United States is:
 1. A process for simultaneously providing bothhigh and low purity oxygen from a double column, double reboilerapparatus, the double columns being high and low pressure columns, saidlow pressure column having a lower and upper reboiler, which processcomprises:a) passing a compressed and cooled feed stream containing atleast oxygen and nitrogen to a main exchanger, wherein the compressedand cooled feed stream is divided into a first fraction and a secondfraction; b) expanding the first fraction in an expander and dividingthe same into a third and fourth fraction; c) further cooling the secondfraction in the main exchanger and expanding at least a fraction of thisstream by an expansion valve into a high pressure column as liquid feedstream; d) introducing the third fraction into a lower reboiler locatedat a bottom portion of the low pressure distillation column, whereby thethird fraction of the feed stream condenses in the reboiler providing afirst reboil for distillation; e) feeding at least a fraction of aresulting liquefied feed stream through either a high or lower pressurecolumn as feed; f) introducing the fourth fraction to the bottom of ahigh pressure column as gaseous feed, whereby a bottom oxygen-richliquid stream and an overhead nitrogen-rich gaseous stream are formed;g) passing the oxygen-rich liquid stream to the low pressure column asfeed; h) condensing the nitrogen-rich gaseous stream against a boilingliquid in an upper reboiler of the low pressure column, located at anintermediate stage, thereby serving as a second reboil for distillation;i) returning a portion of the condensed nitrogen-rich stream to the highpressure column as reflux, and passing at least a fraction of theremaining portion of the nitrogen-rich stream to a top portion of thelower pressure column as reflux, whereby the low pressure columnseparates its feed into bottom stream oxygen products and top streamrich in nitrogen; j) recovering the bottom stream oxygen product as highpurity liquid oxygen; k) drawing from a tray or a stage between the tworeboilers of the low pressure column, the stream-oxygen product, as lowpurity oxygen; and l) warming in the main exchanger the low purityoxygen stream, and the nitrogen-rich stream.
 2. The process of claim 1,wherein the feed stream further contains argon.
 3. The process of claim2, wherein the feed stream is atmospheric air.
 4. The process of claim2, which further comprises recovering argon by means of an argon sidearmcolumn at a tray or a stage between the two oxygen product outlets. 5.The process of claim 1, wherein in step a), said feed stream iscompressed to a pressure of about 6 to 50 bar.
 6. The process of claim5, wherein said feed stream is compressed to a pressure of about 10 to35 bar.